The use of acoustic energy to levitate or position an object within a chamber has many uses, especially in satellites to hold objects under almost zero gravity without touching them with a solid support (containerless processing). One important use is to position a solid object while it is being heated to a temperature at which it melts. Typical sound generators cannot withstand the high temperatures at which many glasses and metals melt. One technique for melting an object is to use an elongated chamber and to heat only one end into which the object is placed, with the other end being maintained at a much lower temperature and with the driver connected to the lower temperature end or cold end of the chamber.
One problem that occurs in the use of acoustic energy to position an object in such a chamber, is to maintain sufficient acoustic pressure at resonant conditions to levitate the object. The velocity of sound increases as the temperature of the gas through which it travels increases, and in a chamber with only one end portion heated there is a variation in the sound velocity. It can be difficult to maintain high acoustic pressure levels while the wave length of sound varies in only one chamber portion. With a variable frequency acoustic generator, it is possible to track the varying resonant frequency. However, because of the nonuniform acoustic conditions in the chamber, the pressure at the hot end would not necessarily be the maximum that could be obtained. The problem can be even more difficult where an acoustic generator is used which generates only a single frequency, or which is much less efficient at frequencies away from a preferred frequency. A system which maximized the pressure in the heated region as well as the maintenance of resonant conditions in a chamber while one portion of the chamber was being heated, would be of considerable value in the acoustic levitation of heated objects.